The present invention relates to an improvement of an adhesive sheet which has a specific uneven structure oh its adhesive surface, and can be adhered to an adherent without trapping unnecessary air between the adhesive surface and the surface of the adherent.
Adhesive sheets are known, which can be bonded or adhered to adherents without trapping unnecessary air between the adhesive surface and the surface of the adherent.
An example of such an adhesive sheet (including an adhesive film) comprises a substrate film and an adhesive layer which is in contact with at least one of the main surfaces of the substrate film, and which has an regularly patterned uneven structure comprising (i) a plurality of projections which will be in contact with an adherent and (ii) grooves which are formed between adjacent projections and will communicate outside when the projections are in contact with the adherent.
This type of the adhesive sheet can allow the trapped air bubbles to escape outside when the adhesive sheet is in contact with an adherent. Thus, the adhesive sheet can be successfully adhered to the adherent without trapping air between the adhesive surface of the sheet and the surface of the adherent.
Such adhesive sheets comprising adhesive layers having such an uneven structure described above are disclosed in, for example, Japanese Patent Publication JP-U (utility model)-3-67043-A.
A conventional method for producing an adhesive sheet will be briefly explained.
First, a liner, an adhesive layer and a substrate film are laminated in this order to provide a laminate comprising these three layers.
Next, an embossing tool is pressed on the liner side of the laminate, and the laminate is processed so that a specific uneven structure is formed on the adhesive layer (adhesive surface). Therefore, the liner is made of a material which can be plastically deformed by above embossing, such as a resin film.
The substrate film of the usual adhesive sheet is also plastically deformed by above embossing, since it comprises a resin film. Thus, embossing marks remain on the main surface of the substrate film on which no adhesive layer is formed, and the appearance of the adhesive sheet deteriorates.
It is also known that a negative structure corresponding to a structure, which will be formed on the surface of an adhesive surface is formed on the surface of a liner. A liquid containing an adhesive polymer is applied on the structural surface of the liner and solidified to form a laminate consisting of the liner and solidified adhesive layer, and then a resin substrate film is laminated on the surface of the adhesive layer to form an adhesive sheet (see, for example, U.S. Pat. No. 5,650,215 (Mazurek et al.)). In this method, the liner and adhesive layer are laminated so that the positive structure on the adhesive surface and the negative structure of the liner are in contact with each other. In this case, no embossing marks remain on the main surface.
However, none of the above prior art teach that the adhesive layers of the adhesive sheets having the uneven structure contain elastic microspheres.
As described above, the adhesive sheets are known, which have no embossing marks on the main surface of the substrate film which is not in contact with the adhesive layer, and which can effectively prevent the deterioration of appearance.
However, with such an adhesive sheet, the uneven structure of the adhesive layer (i.e., adhesive surface) remains after the adhesion of the adhesive sheet to the adherent, and thus the shape of the uneven structure appears on the surface of the substrate film. Therefore, it is difficult to prevent the deterioration of the appearance of the adhesive sheet after the adhesion to the adherent. In particular, with the flexible substrate film, depressions are formed in areas of the substrate film corresponding to the depressed portions of the adhesive layer, when the adhesive sheet are adhered to the adherent, and such depressed portions remain for a relatively long time after the completion of the adhesion.
Thus, an aspect of the present invention provides an adhesive sheet which can maintain a specific uneven structure just after adhesion to an adherent so that bubbles can effectively escape even when the bubbles are trapped, and thereafter, the uneven structure deforms so that deterioration of the appearance can be prevented after adhesion.
The present invention provides an adhesive sheet comprising
(a) a flexible substrate film;
(b) an adhesive layer which is in contact with at least one of the main surfaces of said flexible substrate sheet, and which has an uneven structure comprising (i) a plurality of projections which will be in contact with an adherent and (ii) grooves which are formed between adjacent projections and will communicate outside when said plurality of projections are in contact with the adherent; and
(c) a liner which has, on at least one of its surfaces, a negative structure corresponding to said uneven structure of the adhesive layer, and is laminated on said adhesive layer so that said negative structure and said uneven structure on the surface of the adhesive layer are in contact with each other.
Preferably, in one embodiment, the adhesive layer comprises a crosslinked polymer, and maintains said uneven structure having an initial contact area just after the adhesive sheet is adhered to the adherent. Said uneven structure deforms to form an increased contact area of at least 92% after 48 hours at 25xc2x0 C. from the adhesion of the sheet to the adherent. In a second embodiment, said adhesive layer comprises a matrix component containing an adhesive polymer and elastic microspheres, which are dispersed in said matrix component. The elastic microspheres have an average diameter of 10 to 100 xcexcm as described below.
Further embodiments are disclosed below.
The adhesive sheet of the first embodiment according to the present invention has the following characteristics:
(a) the adhesive layer contains a crosslinked polymer, and
(b) the adhesive sheet maintains-an uneven structure just after a liner is peeled off and the adhesive sheet is adhered to the adherent. The uneven structure deforms and thus a contact area increases to at least 92% after 48 hours at 25xc2x0 C. from the adhesion of the sheet to the adherent.
The above characteristic (b) is easily achieved due to the shape memory of the adhesive layer containing the crosslinked polymer. That is, such a characteristic is obtained by allowing the adhesive layer to memorize a flat shape having no uneven structure as the first shape, and then shaping the adhesive sheet to impart the specific uneven structure as the second shape to the adhesive layer. The easy and effective method for forming such an adhesive sheet will be explained below. In this case, the above liner, that is, the liner which has, on at least one of its surfaces, the negative structure corresponding to the uneven structure of the adhesive layer, and is laminated on the adhesive layer so that the negative structure and the uneven structure on the surface of the adhesive layer are in contact with each other, functions as a negative mold for imparting the above second shape (positive shape), and also as a protective film for maintaining the second shape of the adhesive layer during storage (before use).
The adhesive layer contains an adhesive polymer, and has tackiness at room temperature (about 25xc2x0 C.). The crosslinked polymer may be the above adhesive polymer which has been crosslinked, or a separately added non-adhesive crosslinked polymer.
The polymer is usually crosslinked with a crosslinking agent. The amounts of the components are suitably selected in a range in which the adhesion force of the adhesive layer is not lower than a specified level, and the uneven structure deforms within a specific period of time so that the contact area increases to 92% or larger. For example, the adhesive layer may be formed from a liquid (coating composition) for the adhesive layer containing 0.01 to 5 wt. parts of the crosslinking agent per 100 parts of the adhesive polymer contained in the adhesive layer.
To accelerate the crosslinking of the polymer, an energy such as heat and radiation can be preferably used. The polymer can be crosslinked by directly reacting reactive sites of the polymer chains.
In the adhesive sheet of the second embodiment, the adhesive layer comprises (1) a matrix component containing an adhesive polymer and (2) elastic microspheres dispersed in the matrix component, wherein the elastic microspheres have an average diameter of 10 to 100 xcexcm.
The elastic microspheres assist in the recovery of the original shape of the adhesive layer after the adhesive layer is deformed from the originally imparted shape. For example, when the adhesive sheet is produced by forming an adhesive layer on a structural surface of a liner by the application and drying of the liquid for the adhesive layer, and then laminating a substrate film, an adhesive layer is formed, which has (i) an uneven structure corresponding to the structural surface of the liner on the adhesive surface and (ii) a flat surface which is in contact with the substrate film. When such an adhesive sheet is adhered to an adherent and then pressed from the substrate film side against the adherent, the adhesive layer deforms, and depressed portions are formed on the surface of the substrate film at areas corresponding to the depressed portions of the adhesive layer (i.e., on the liner side of the adhesive surface). However, the deformation of the adhesive layer is accompanied with the elastic deformation of the elastic microspheres, and thus, the adhesive layer recovers the original shape because of the shape recovering function of the elastic microspheres.
Such a shape recovery effect can be expected when the adhesive sheet is produced by the following method. The method comprises the following steps:
(1) a layer of an adhesive agent having a substantially flat adhesive surface is formed on at least one surface of a substrate film, wherein the adhesive agent comprises a matrix component containing an adhesive polymer and elastic microspheres dispersed in the matrix component, and
(2) the adhesive surface of the adhesive agent layer and a structural surface of a liner corresponding to a negative uneven structure are allowed in contact with each other to form an adhesive layer having a positive uneven structure corresponding to the negative structure of the liner.
This method can produce an adhesive sheet, which effectively and easily prevents deterioration of the appearance of the adhesive sheet after adhesion.
The uneven structure of the adhesive layer is not limited, insofar as the effects of the present invention are not impaired.
The uneven structure comprises projections each having a horizontal cross-sectional area (i.e., a cross-section in parallel with the adhesive surface) having a polygonal (e.g. square), round or similar shape, and grooves, which are continuously formed to surround the projections.
The shape of a vertical section of each projection may be square such as trapezoidal or rectangular, semicircular or the like.
The height of the projections (and the depth of the groove) is usually between 5 and 200 xcexcm, preferably between 10 and 100 xcexcm. When the height of the projections is too low, it may be difficult to remove bubbles trapped between the adhesive surface and adherent surface. When the height of the projections is too high, the appearance of the adhesive sheet may be impaired after the completion of the adhesion.
The maximum width of the projections (i.e., the maximum size in the horizontal direction) is usually between 0.1 and 10 mm. The maximum width of the groove (i.e., the maximum distance between adjacent projections) is usually between 0.05 and 1 mm.
The projections are preferably arranged uniformly. For example, each projection is placed substantially at the center of each square of a check pattern. In this case, the grooves are formed along the lines constituting the check pattern, and have one or more openings so that at least one groove, preferably a plurality of grooves can communicate with the outside.
The sizes of a plurality of projections and grooves are preferably all the same.
Herein, the term xe2x80x9celastic microspherexe2x80x9d, which may be referred to as simply xe2x80x9cmicrospherexe2x80x9d, is intended to mean a material, which exhibits rubbery elasticity as a whole.
The average diameter of the microspheres is usually between 10 and 100 xcexcm, preferably between 20 and 80 xcexcm. When the average diameter is less than 10 xcexcm, the above shape recovery effect is not attained. When the average diameter exceeds 100 xcexcm, the unevenness due to the presence of the microspheres may be recognized from the surface of the substrate film.
The average diameter is derived by measuring diameters of 1000 microspheres using an image analyzer with an optical microscope and calculating an average value according to the following equation (1):
average diameter (xcexcm)=xcexa3(di4xc3x97ni)/xcexa3(di3xc3x97ni)xe2x80x83xe2x80x83(1)
wherein di is a diameter (xcexcm) of a microsphere having an i-th largest diameter, and ni is the number of microspheres having the diameter di.
The microsphere may be solid or hollow having at least one void therein.
The microspheres preferably comprise polyacrylate (acrylic copolymer), since its rubbery elasticity and tackiness are easily controlled.
The compressive modulus of the microspheres is preferably between 1xc3x97104 and 1xc3x97106 dyne/cm2. In this range, the shape recovery effect of the microspheres is good.
The compressive modulus is measured using a RSA II viscoelastic spectrometer (manufactured by RHEOMETRIX) at 20xc2x0 C. That is, the temperature dependency of the modulus is measured by changing a temperature from xe2x88x9280xc2x0 C. to 150xc2x0 C. while applying a compression strain having a frequency of 1 rad/sec. A measured value at 20xc2x0 C. is used as the compressive modulus.
The microspheres may be tacky or non-tacky. In general, the microspheres contain a crosslinked polymer.
The elastic microspheres may be produced by any known method such as suspension polymerization, emulsion polymerization, seed polymerization, and the like. A method for producing polyacrylate microspheres by emulsion polymerization will be briefly explained by way of example.
In a reactor equipped with a mechanical stirrer, deionized water, an acrylic monomer, a radical polymerization initiator, and other optional additives are added, and the interior of the reactor is purged with an inert gas. Then, the reactor is heated to a specific temperature to initiate polymerization of the monomer while stirring. In general, the stirring rate is from 10 to 700 rpm, and the reaction temperature is from 30 to 120xc2x0 C. The reaction time is usually from several hours to ten hours.
As the acrylic monomer, a mixture of an alkyl acrylate (e.g. isooctyl acrylate, 2-ethylhexyl acrylate, isononyl acrylate, etc.) and an acrylic unsaturated acid (e.g. acrylic acid, methacrylic acid, itaconic acid, maleic acid, etc.) may be used. A weight ratio of the alkyl acrylate to the acrylic unsaturated acid is preferably from 99:1 to 90:10. When the amount of the alkyl acrylate is too small, the tackiness is lowered. When the amount of the alkyl acrylate is too large, the rubbery elasticity decreases. In either case, the adhesion force to the uneven surface tends to decrease. It is possible to crosslink the polyacrylate by the addition of a crosslinking agent comprising a bifunctional acrylate such as divinyl benzene 1,4-dibutyl-2-diacrylate to the above mixture.
The production method of the polyacrylate microspheres is disclosed in, for example, U.S. Pat. No. 4,994,322.
The produced microspheres are usually recovered by filtration and used. An aqueous dispersion containing the microspheres following the reaction may also be used. The tacky polymer is added to the dispersion to obtain the adhesive coating composition.
The term xe2x80x9cadhesive polymerxe2x80x9d used herein means a polymer, which exhibits adhesion at room temperature and can be used as a pressure sensitive adhesive. Polyacrylate, polyurethane, polyolefin, polyester, and the like can be used as such a polymer. A tackifier may be used in combination with the adhesive polymer, as in the case of conventional pressure sensitive adhesives. The adhesive polymer may be a polymer, which can be cured with heat or radiation after the uneven structure of the adhesive layer deforms and the appearance of the adhesive sheet is improved insofar as the effects of the present invention are not impaired.
The molecular weight of the adhesive polymer may be in a range in which the desired viscosity is achieved. In general, the weight average molecular weight of the adhesive polymer is between 10,000 and 100,000.
The adhesive polymer can be used in the form of a solution (in which the polymer is dissolved in a solvent) or an emulsion (in which the polymer is dispersed in a solvent). The solution and emulsion may be used in admixture.
When the adhesive polymer is crosslinked, a crosslinking agent comprising an isocyanate compound, a melamine compound, a poly(meth)acrylate compound, an epoxy compound, an amide compound, and the like can be added. When the adhesive layer contains a non-adhesive polymer, the polymers can be crosslinked with such a crosslinking agent.
In either the first or second embodiment, the thickness of the adhesive layer is not limited insofar as the effects of the present invention are not impaired, and is usually between 10 and 200 xcexcm.
In the second embodiment, the amounts of the microspheres and adhesive polymer are selected so that the amount of the microspheres is usually between 5 and 500 wt. parts, preferably between 20 and 400 wt. parts, per 100 wt. parts of the adhesive polymer. When the amount of the microspheres is less than 5 wt. parts, the above shape recovery effect may not be attained. When such an amount exceeds 500 wt. parts, the adhesion force tends to decrease.
As the substrate film, any substrate film of conventional adhesive films may be used. For example, a paper sheet, a metal film, a plastic film and so on can be used. As the plastic, a synthetic polymer such as polyvinyl chloride, polyester, polyurethane, polyacrylate, polyolefin, etc. can be used. The thickness of the film is usually from 10 to 1500 xcexcm.
In one embodiment, the thickness of the film is between 10 and 500 xcexcm. The surface of the substrate film on which the adhesive layer is formed may be treated to improve the adhesion force between the substrate film and the adhesive layer.
The contact area of the adhesive layer is defined as a ratio of a contacted area between the adhesive layer and a flat surface of a glass plate, when the adhesive sheet is adhered to the flat surface of the glass plate using the procedure described below.
The flat surface of a glass plate, such as a slide glass, and the adhesive layer of the adhesive sheet are adhered, and pressed by reciprocating a roller of 2 kg in weight over the film one time to obtain a sample. The width of the adhesive sheet is 2 cm so that a linear pressure is 1 kg/cm. The roller is reciprocated in the longitudinal direction of the sheet.
When the glass plate is illuminated by a white light from the glass surface on which the adhesive film was not adhered, the reflected light is observed through a polarized light filter. Domains in which the projecting adhesive parts and the glass surface are in contact with each other are seen dark, while noncontact domains are seen white. The image is photographed, and a ratio of the total area of the contact domains to the area of the whole observed field (corresponding to the maximum possible contact area) is expressed as a xe2x80x9cpercentagexe2x80x9d, which is used as the xe2x80x9ccontact area ratexe2x80x9d. Such procedures can be carried out using an optical microscope equipped with a Polaroid camera. Here, the area of the whole observed field is usually about 1 cm2. The surface roughness, Ra, of the used glass plate is 0.1 xcexcm or less.
With the first embodiment, the observed contact area is usually 92% or larger, preferably 95% or larger, more preferably 98% or larger, after the adhered sheet is maintained at 25xc2x0 C. for 48 hours and the uneven structure deforms.
The adhesive sheet of the first embodiment may be formed by applying a coating liquid for the adhesive layer on one or both of the surfaces of the substrate film, drying the applied coating liquid to form an adhesive layer, and pressing an uneven structural surface of a liner against the flat outer surface of the adhesive layer.
The coating liquid for the adhesive layer can be prepared by mixing the adhesive polymer, crosslinking agent, solvent and optional additives with a mixing apparatus such as a homomixer, a planetary mixer, etc. to disperse or dissolve all the components uniformly.
The prepared coating liquid is coated on the substrate film, and dried to form the adhesive layer. The coating liquid can be coated by any conventional coating means such as a knife coater, a roll coater, a die coater, a bar coater and the like. The coated liquid is usually dried at a temperature between 60 and 180xc2x0 C. The drying time is usually between several ten seconds to several minutes.
Water or organic solvents can be used as solvents. Cosolvents, which are partly miscible with water, may be added. Examples of useful co-solvents are alkyleneglycol monoalkyl ether esters such as 3-methyl-3-methoxybutyl acetate.
The coating liquid may contain conventional additives such as a viscosity modifier, an anti-foaming agent, a leveling agent, a UV light absorber, an antioxidant, a pigment, an anti-fungus agent, etc. In addition, the coating liquid may contain inorganic particles (e.g. glass beads, etc.) or organic particles other than the elastic microspheres, insofar as the effects of the present invention are not impaired.
The liner is usually made of a paper sheet, a plastic film, or a laminate film thereof. For example, an embossing tool having a positive shape is pressed onto a flat surface of the film, and an uneven structure corresponding to the positive shape is transferred to the film surface. During embossing, the tool can be heated. The above positive shape has the same shape and size as the uneven structure to be formed on the adhesive surface.
Alternatively, a flowable material comprising plastics is poured on a mold having a positive structure, and solidified. Then, the mold is removed, and a liner having an uneven structure corresponding to the positive shape is obtained.
The uneven structural surface of the liner may be treated with, for example, silicones to impart releasability. When the plastics are polyolefins, the releasing treatment may be omitted.
Alternatively, a marketed adhesive sheet is purchased, and the original liner is replaced with the above uneven liner. Then, the adhesive layer and uneven liner are pressed to impart the uneven structure to the adhesive layer.
The adhesive sheet of the second embodiment can be produced by applying the above coating liquid containing elastic microspheres onto the uneven structural surface of the liner to form an adhesive layer, and then laminating a substrate film on the outer surface of the adhesive layer. The preparation of the coating liquid and coating method are the same as those described in the first embodiment. The liner can be the same as that used in the first embodiment.
The adhesive surface having an uneven structure may be formed by pressing an uneven liner onto an adhesive layer having a substantially flat adhesive surface, which has been formed on a substrate film.
The adhesive sheet of the present invention can be used as a decorative sheet and adhered to walls, floors and ceilings of buildings, signboards, etc.